1. Field of the Invention
This invention relates to an infrared focal plane array detector and a method of producing the same, and more particularly to an infrared focal plane array detector which is less likely to be influenced by an offset signal and a temperature drift and a method of producing the same.
2. Description of the Related Art
A bolometer detector has a resistance value which varies in response to a temperature variation, and is used widely for detection of infrared rays with the characteristic made most of. A mechanism of operation of a conventional infrared focal plane array detector which employs a bolometer detector and is an example of an application is described with reference to FIG. 1.
A plurality of unit cells 400 each including one bolometer detector 401 are arranged in a two-dimensional array in FPA (FOCAL PLANE ARRAY) 417 such that they may form columns and rows. Bolometer detector 401 in each unit cell 400 is connected to ground terminal 403 through unit cell selecting transistor or switch 402. The gate electrodes of unit cell selecting switches 402 are controlled by vertical shift register 410 through horizontal lines 406 and are selected such that all of the unit cell selecting switches 402 connected to one horizontal lines 406 may be put into a closed state. Consequently, on each vertical line 404, bolometer detector 401 on one of the unit cells 400 is selectively connected to ground terminal 403.
Vertical lines 404 are connected to an on-chip amplifier 407 through vertical line selection switches 405 and over to output line 421. Vertical line selection switches 405 are controlled by horizontal shift register 409 such that one of vertical lines 404 may be connected to an on-chip amplifier 407 at a given time.
A light receiver of each bolometer detector 401 is in a high thermally insulating state with respect to asubstrate, and consequently, thermal energy by infrared radiation is temporarily stored into bolometer detector 401. As a result, the temperature of bolometer detector 401 rises, and a resistance variation corresponding to the temperature variation appears with bolometer detector 401. Temperature information of an image pickup object can be obtained by externally reading out the resistance variation through output terminal 408.
For the readout circuit, for example, integration circuit 440 is used. In integration circuit 440, electric current flowing through integration transistor 441 while a fixed voltage is applied to bolometer detector 401 for a fixed time is integrated by means of integrating capacitor 442. Actually, integrating capacitor 442 connected in series to bolometer detector 401 is charged up to a predetermined voltage in advance, and then, after integrating capacitor 442 is energized with the fixed voltage for a fixed time, the remaining voltage of integrating capacitor 442 is read out. Integration transistor 441 acts to keep the voltage of output terminal 408 fixed even if the terminal voltage of integrating capacitor 442 varies during the integration operation. Accordingly, the remaining voltage of integrating capacitor 442 after the integration operation comes to an end depends upon the resistance value of bolometer detector 401 within the integration period and includes information regarding the amount of heat radiation received from the image pickup object. Information of the infrared radiation amount can be read out electrically in this manner.
The infrared focal plane array detector which employs a bolometer element described above, however, has a problem in that offset components of the output signal are so high that a sufficient gain cannot be obtained. A principal part of the offset components of the signal arises from a large variation of the bolometer resistance by a temperature rise caused by joule heating of the bolometer detector within the integration period. Although the offset components occupy the greater part of the output signal, it does not include temperature information of the object at all. Therefore, the dynamic range of an amplifier cannot be used effectively, and the signal gain cannot be raised. If the integration time is increased, then the temperature resolution is augmented as much. However, since this increases also the offset amount simultaneously, a sufficient integration time cannot be assured.
The infrared focal plane array detector which employs a bolometer described above has another significant problem in temperature drift. Although each bolometer detector is thermally isolated from the substrate, actually a little heat exchange with the substrate is present through signal readout wires. Therefore, if the ambient temperature varies and the substrate temperature varies, then the element temperature of the bolometer detector varies with a long time constant. This makes a cause of a temperature drift. In order to prevent this, FPN (FIXED PATTERN NOISE) correction must be performed frequently during operation, and this significantly deteriorates the operability of an infrared camera which employs the infrared detector.
It is an object of the present invention to provide a new infrared focal plane array detector which eliminates the drawbacks of the prior art described above and particularly removes an offset and eliminates an influence of a temperature drift thereby to augment the accuracy in detection of infrared rays and a method of producing the infrared focal plane array detector.
An-infrared focal plane array detector of the present invention comprises first bolometer detectors and at least one set of second bolometer detectors and a plurality of third bolometer detectors being provided on the same substrate. First bolometer detectors have, between the first bolometer detectors and a substrate, a thermal isolation structure for thermally isolating the first bolometer detectors and the substrate from each other and disposed in an infrared detection area on the substrate for receiving infrared rays incoming thereto. Second bolometer detectors have a thermal isolation structure between the second bolometer detectors and the substrate for thermally isolating the second bolometer detectors from the substrate and are so structured as to receive no infrared rays incoming to the infrared focal plane array detector. Third bolometer detectors are directly formed on the substrate. The bolometer detectors forming a circuit for removing an offset in output signals therefrom and reducing a temperature drift thereof.
Preferably, an arbitrary one of the first bolometers disposed in the infrared detection area and one of the third bolometer detectors disposed on the same column as the first bolometer detector are connected to each other at a first junction, and one of the second bolometer detectors disposed on the same row as the first bolometer detector and another one of the third bolometer detectors disposed on the same column as the second bolometer detector and different from the third bolometer detector are connected to each other at a second junction, and further, a bridge circuit is formed from the four bolometer detectors and the first junction and the second junction form midpoints for detecting a difference in voltage of the bridge circuit. Alternatively, the infrared focal plane array detector may be constructed such that an arbitrary one of the first bolometer detectors disposed in the infrared detection area is connected to one of the second bolometer detectors disposed on the same column as the first bolometer detector, and one of the third bolometer detectors disposed on the same row as the first bolometer detector is connected to another one of the third bolometer detectors disposed on the same column as the third bolometer detector, and further, a bridge circuit is formed from the four bolometer detectors and a junction between the first bolometer detector and the second bolometer detector and another junction between the two third bolometer detectors form midpoints for detecting a difference in voltage of the bridge circuit.
The infrared focal plane array detector may be constructed such that the infrared focal plane array detector includes a plurality of the second bolometer detectors, and plurality of switching element is provided for selectively using one of the plurality of second bolometer detectors in the bridge circuit, or such that the infrared focal plane array detector includes a plurality of the third bolometer detectors, and plurality of switching elements is provided for selectively using one of the plurality of third bolometer detectors in the bridge circuit.
Infrared ray interception means for the second bolometer detectors may be an infrared ray interception layer provided on an infrared ray incoming side of the second bolometer detector or may alternatively be an infrared ray reflection film provided on the second bolometer detectors on the substrate.
A switching transistor for selecting the first bolometer detectors and a differential amplifier for detecting a potential difference between the midpoints of the bridge circuit may be formed on the substrate on which the bolometer detectors are formed.
A method of producing an infrared focal plane array detector of the present invention comprises a first step of forming switching transistors for selecting bolometer detectors on a substrate, a second step of forming a sacrificial layer on the substrate, a third step of forming a film made of a bolometer material and a protective film on the sacrificial layer such that the film may be held between the protective film, a fourth step of etching the film made of the bolometer material into a predetermined shape and covering the film with the protective film, a fifth step of forming, in accordance with an application of the bolometer detectors, an infrared ray absorption film for the first bolometer detectors and the third bolometer detectors or one of an infrared ray reflection film and a light interception film for the second bolometer detectors on the protective film and patterning the infrared absorption film, infrared reflection film or light interception film into a predetermined shape, a sixth step of removing the sacrificial layer except that at a predetermined location including the third bolometer detectors to form a space, and a seventh step of containing the space under a vacuum state.
Preferably, three kinds of bolometer detectors are produced from the same material by the same process. The first step may include formation of a differential amplifier to which midpoint potentials of a bridge circuit formed from the three kinds of bolometer detectors are inputted.
By employing the structure described above, an influence of a temperature drift can be eliminated, and an offset component arising from a dispersion in in-plane characteristic of the bolometers and an offset component arising from joule heat generation upon signal reading out can be removed. Consequently, an infrared focal plane array detector which allows detection of infrared rays with a high degree of accuracy and is suitable for an infrared camera can be provided.
The above and other objects, features and advantages of the present invention will become apparent from the following descriptions based on the accompanying drawings which illustrate an example of a preferred embodiment of the present invention.